1. Field of the Invention
This invention relates generally to systems and methods for control of video displays. More particularly, this invention relates systems and methods that control of contrast and brightness of a video display.
2. Description of Related Art
Color video displays receive digital or analog signals that indicate the magnitude of the magnitude of the luminance of the primary colors employed by the display to create a color image. There are numerous standards that are known in the art that generate these signals for exciting materials within the display responsible for generating the light of the image. In cathode ray tubes (CRT), the light is emitted by phosphors deposited on a surface of the CRT. An electron gun is stimulated by the input analog signal to create the appropriate magnitude of emission of the light to create the color components of the image. In a liquid crystal display (LCD), a voltage is applied to a polymeric material which is twisted to cause polarization of light to be rotated. The level of the voltage determines the amount of rotation of the polarization and thus acts as to adjust the luminance of the light transfer through the LCD or reflected from the LCD. The standards for computer displays, as is known in the art, are particularly designed for use with CRT type displays and must be adapted for use with LCD displays.
Refer to FIG. 1 for a discussion of a circuit of the prior art employed to adapt a video signal to the necessary levels for driving an LCD panel. The video signal 5 is the analog voltage level indicating the luminance level that the display is to emit. The video signal 5 is received by a digital to analog-to-digital converter 10 and converted to a sampled digital number indicating the magnitude of the voltage of the video signal 5. The sampled digital value of the video signal is transferred from the analog-to-digital converter 10 to a scaling circuit 15. The scaling circuit 15 adjusts or normalizes the digital value of the video signal such that the range of the video signal encompasses the range of the voltage required for driving the LCD panel 45. The gain control signal 25 is a digital signal indicating the desired contrast level of the LCD panel. The scaled digital value of the video signal from the scaling circuit 15 and the gain control signal 25 are the inputs to a multiplier circuit 20. The gain control signal 25 being a multiplying factor for the scaled digital value of the video signal. The output of the multiplier circuit 20 being the product of the scaled digital value of the video signal and the gain control signal 25 is the input to the adder circuit 30. The offset control signal 35 is digital value indicating the desired brightness of the LCD panel 45. The offset control signal is added to the scaled and gain adjusted video signal to provide the desired level of luminance for the LCD panel. This value is really a pointer to a position within the gamma correction mapping table 40 representing the desired luminance of the LCD panel. It is known in the art that the function 42 of the relative luminance of the LCD panel to the drive voltage necessary to achieve the luminance is not linear. Further, providing a “Look-Up” table 40 of entries for this function is requires less computation resources and circuitry than direct calculation of the function. In the “Look-Up” table 40, the magnitude of the video signal for the desired relative luminance and the corresponding table entry is the value of the drive voltage required for the LCD panel 45 to emit light at the desired luminance.
The drive level (Z) is thus determined by the function:Z=G_orig((x*a)+b)where:                Z is the magnitude of the voltage necessary to drive the LCD panel 45.        x is the voltage level of the video signal 40.        a is the magnitude of the gain control signal 25.        b is magnitude of the offset signal 35.        G_orig is the function 42 as mapped in the gamma correction mapping table 45.        
Refer now to FIG. 2 for a discussion of the operation of the circuit to adapt the video 5 signal for operation with the LCD panel 45. The digital value of the video signal 5 is applied to the multiplier circuit 20 to generate the scaled values 22 of the video signal. The scaling being the gain value (a). The adder circuit then adds the offset value (b) to generate the mapping values 32. The mapping values 32 act as pointers (Y) to the gamma correction mapping table 40. The values of the gamma function (G_orig) as recorded in the gamma correction mapping table 40 are extracted and applied as the drive signal (Z) 44 for the LCD panel 45.
The circuitry as described requires a physical multiplier and a physical adder. This adds complexity and consequently cost to a video display system. Further, the additional circuitry impacts performance of the video display system, particularly with higher resolution displays.
U.S. Pat. No. 5,796,384 (Kim) describes a gamma correction circuit of a liquid crystal display using a memory device. The gamma correction circuit compensates for the non-linear characteristics of gamma. The gamma correction circuit includes a memory device for storing programmed data. The programmed data is used to make gamma constant and to make the light transmissivity vary linearly with the input image data. The programmed data may be changed according to the characteristics of the liquid crystal being driven.
U.S. Pat No. 5,987,167 (Inoue) illustrates a color image display that includes an input color characteristic obtaining unit for obtaining input color characteristic data simultaneously with input of image data. The display has an input color characteristic data storage unit for storing the obtained input characteristic data. The display includes an image display device such as a CRT. A display color characteristic data storage unit stores the color characteristic of the image display device. A color transformation table generator creates a color transformation table and a color transforming circuit provides the color-transformation of the image data from selected color transformation tables.
U.S. Pat. No. 6,388,648 (Clifton, et al.) teaches an LCD projection unit. The LCD projection unit employs a luminance and color balance system. The luminance and color balance system employs an LCD array characterization lookup table that stores multiple sets of luminance and gamma correction values that are user selectable to control luminance and color balance. The lookup table values are determined by measuring the transfer function of the LCD array in the projection unit for each of a set of red (“R”), green (“G”), and blue (“B”) input data values. The resulting S-curve responses are converted to corresponding sets of gamma responses and scaling the gamma responses to generate multiple R, G, and B families of luminance and gamma corrected values. Color balance is adjusted by selecting the particular R, G, and B families of luminance and gamma corrected values that cause the LCD projection unit to match a predetermined ratio of R, G, and B luminance values. Luminance is adjusted by selecting sets of families of R, G, and B, luminance values that maintain the color balance ratio while causing the LCD to transmit the desired overall luminance. The primary colors are adjusted by a method of color mixing implemented by a mathematical matrix algorithm that generates color modification coefficients for a color space conversion circuit. The primary color matching algorithm involves measuring the intrinsic colors coordinates of the primaries, determining a set of predetermined target coordinates, and performing matrix operations to calculate the coefficients used in the color space conversion circuit to convert the measured to the target coordinates, thereby matching the primary colors.
U.S. Pat. No. 6,271,822 (Chiang) describes a digital LCD driving circuit. The LCD driving circuit drives an LCD to display video images. The LCD driving circuit includes a digital gamma-correction and inversion circuit for performing a digital gamma-correction process on the digitized video signal and then performing a polarity inversion process on selected lines of the video signal. The digital gamma-correction and inversion circuit is coupled directly to a display memory unit so as to fetch the digitized video signal directly from the display memory unit. A digital-to-analog converter is coupled to the digital gamma-correction and inversion circuit for converting the digital output of the digital gamma-correction and inversion circuit into analog form. Further, the LCD driving circuit comprises an LCD timing control circuit, which is coupled to receive a plurality of video control signals associated with the digitized video signal, for converting the video control signals into an LCD timing control signal to control the LCD to display the video signal. Still further, the LCD driving circuit has a Pulse Width Modulator and shutdown circuit for supplying power to the LCD and shutting down the LCD when the LCD has been idle for a preset period.